The object of the present invention is a method for switching the striking stroke of a striking piston of a percussion device actuated by a pressurized incompressible fluid, and a percussion device for implementing this method.
Percussion devices actuated by a pressurized incompressible fluid are supplied with fluid in such a way that the resultant of the hydraulic forces applying successively on the striking piston moves the latter alternately in one direction then the other.
In such devices, the striking piston moves alternately inside a cylinder in which at least two antagonistic chambers with different sections are arranged. The one, constantly supplied with pressurized fluid, called lower chamber, ensures the upstroke of the striking piston, and the other one, antagonistic, with a larger section, called upper chamber, is alternately supplied with pressurized fluid during the stroke of the striking piston and connected to the low-pressure return circuit of the device during the upstroke of the striking piston. It is also known that, if the tool is not properly pressed on the material to be destroyed or if the material is too soft, the device will tend to perform “empty blows” on the tool, these blows are very destructive to the tool and the device itself. The percussion devices are thus generally provided also with a chamber, called brake chamber, which is used to hydraulically stop the stroke of the striking piston when the tool is not pressed on the material to be destroyed. The presence of such a brake chamber allows to avoid direct impacts between the striking piston and the cylinder. This brake chamber can be advantageously arranged in the extension of the lower chamber.
It is known that for a given power of the device, expressed by the product of the value of the striking frequency and the value of the energy per blow, when the device works on a homogeneous firm ground, it is preferable to favor energy per blow over the frequency to obtain optimal productivity.
On the contrary, for the device to work on a soft ground, it is advantageous to reduce the energy per blow, and consequently increase the striking frequency.
The energy per blow is the kinetic energy given to the striking piston, and depends on the striking stroke and on the supply pressure. To adjust the striking frequency and the energy per blow suited to the hardness of a given ground, there are several known solutions described in documents EP 0 214 064, EP 0 256 955, EP 0 715 932 and FR 2 902 684 on behalf of the Applicant.
The document EP 0 214 064 describes a device which allows to obtain automatic adjustment of the percussion parameters, thanks to the presence in the cylinder of the device of a channel supplied with fluid according to the position of the striking piston after the impact and the possible rebound of the latter on the tool.
The document EP 0 256 955 describes a device which allows to obtain the same result, depending on the pressure variations in the upper chamber and the lower chamber, consecutive to the effect of the rebound of the striking piston on the tool, thanks to the presence of a hydraulic element which is sensitive to these variations.
The document EP 0 715 932 describes a simplified system that can equip low and medium power devices. This system consists, during the phase of the rebound of the striking piston consecutive to the impact, in determining the possible existence of an instantaneous flow rate of a fluid flowing from the upper chamber to the supply circuit, and using this signal to control the percussion parameters, such as the striking pressure or the frequency of the device.
The document FR 2 902 684 describes a percussion device in which, when the striking piston penetrates the brake chamber, pressurizing the fluid contained in the brake chamber allows to control a slide acting on the stroke of the striking piston, thanks to a channel opening into the brake chamber.
When the devices described in the aforementioned documents are used in operating sequences with very short operation interruptions and in firm ground, it is not uncommon that, at start-up of the device, the impact energy applied to the striking piston is high, which can cause “empty blows” on the tool, and excessive fatigue of the latter and of the keys holding it.